Automatic survey instruments of the present type comprise a telescope system where received light can be divide into separate channels for range-finding, tracking and viewing or manual aiming. The transmitter beam for the range-finder and for the tracker function are preferably coaxial to the receiver optics.
The light reflected from the target object, and received at the instrument is divided, depending upon the purpose, in light components of different wavelengths such as the light for tracking, range finding and visible light for viewing. By using the tracking light and range finding light thus divided, range finding and automatic tracking may be performed.
Such instruments, using more than one channel, according to the prior art, uses different types of prisms in the receiver with or without dichroic coatings for splitting/separating the beams into the different channels.
The word dichroic in this context refers to dichroic mirrors/prisms or coatings, which exhibit selective reflection and transmission of light as a function of wave-length regardless of its plane of vibration. A dichroic mirror thus selectively reflects light according to its wave-length and transmits light having other wavelengths. The use of prisms, i.e. cubic prisms primarily requires long building lengths, problems in providing needed filters within the prisms and inherently high costs, in that the costs for producing the prisms are high.
Belonging to the related art are e.g. EP-1 081 459 and EP-0 987 517. In these discussions of problems related to the use of prisms can be found. In EP-0 987 517 e.g. the arrangement includes several prisms in which two beams having different wavelengths are separated using two dichroic prisms. In EP-1 081 459 prisms are disclosed, but also one dichroic plate is used for separating beams with different wavelengths; however, that plate is arranged perpendicular to the optical axis of the instrument, thus reflecting the light in the coaxial direction, while allowing other wavelengths to pass through the plate.
The use of prisms, however, introduces certain problems. Drawbacks are the fact the glass extends the optical path so the building length increases, the size of the prisms become large and the prisms are heavy. The requirements on parallelity between prisms surfaces are very high and making a dichroic filter within the glass is harder than on a surface between air and glass. All this means that the prisms become large and expensive and the telescope becomes larger and heavier. More advanced prism constructions can reduce building length but the costs further increases for providing the prisms and the complicated structure resulting therefrom. Aditionally, the prisms make the apparatus larger and heavier causing other problems as increased power consumption, as when automatically tracking an object.
Examples of wavelengths used for visible light are in the range of 400 nm to 650 nm for collimating purposes. For range-finding visible light at 660 nm or IR light at 850 nm can be used and for tracking IR light at 785 nm. These wavelength ranges are generally termed channels of a particular type such as visual channel, tracking channel etc.